When tumor cells are eliminated by the immune system the identification of tumor-associated antigens (TAA) by components of the immune system plays a pivotable role. This mechanism is based on the fact that there exist qualitative or quantitative differences between tumor cells and normal cells. To induce an anti-tumor-response, the tumor cells have to express antigens which induce an immune response being sufficient for the elimination of the tumor.
In particular, CD8-expressing cytotoxic T-lymphocytes (in the following CTL) are involved in rejection of tumors. To induce such an immune reaction by cytotoxic T-cells foreign proteins/peptides have to be presented to T-cells. Antigens are recognized as peptide fragments by T-cells only if they are presented by MHC-molecules on cell surfaces. These MHC (“major histocompatibility complex”) molecules are peptide receptors which normally bind peptides intracellularly and transport them to the cell surface. This complex of peptide and MHC-molecule is recognized by T-cells. Human MHC-molecules are also designated as human leukocyte antigens (HLA).
There are two classes of MHC-molecules: MHC class-I-molecules, which are present on most cells having a nucleus, present peptides generated by degradation of endogenous proteins. MHC class-II-molecules are present on professional antigen-presenting cells (APC) only and present peptides of exogenous proteins, which are taken up and processed by APC during endocytosis. Peptide/MHC-class-I complexes are recognized by CD8-positive cytotoxic T-lymphocytes, peptide/MHC class-II-complexes are recognized by CD4 helper T-cells.
In order to induce a cellular immune response a peptide has to bind to a MHC-molecule. This action depends on the allele of the MHC-molecule and on the amino acid sequence of the peptide. MHC-class-I-binding peptides, being—as a general rule—of 8 to 10 residues in length, comprise two conserved residues (“anchor”) in their sequence, that engage complementary pockets located in the MHC-molecule.
In order for the immune system to induce an effective CTL-response directed against tumor-associated peptides, these peptides have not only to be able to bind to specific MHC-class-1-molecules expressed by tumor cells but have also to be able to be recognized by T-cells having specific T-cell receptors (TCR).
When developing a tumor vaccine a main aim is to identify and to characterize tumor-associated antigens which are recognized by CD8+ CTL.
The antigens—or their epitopes, respectively—which are recognized by tumor-specific cytotoxic T-lymphocytes can be molecules of all classes of proteins, such an enzymes, receptors, transcription factors, etc. Another important class of tumor-associated antigens are tissue-specific structures such as the cancer-testis antigens, which are expressed in various kinds of tumors and healthy testis tissue.
In order for the T-lymphocytes to identify proteins as tumor-specific antigens and in order to use them in therapy, certain requirements have to be met: The antigen has to be expressed mainly by tumor cells and not by normal cells or at least to a minor extent as in tumors. Further, it is desirable if the specific antigen is present not only in one kind of tumor but also in other kinds in high concentrations. Further, the presence of epitopes in the amino acid sequence of the antigen is essentially since those peptides derived from tumor-associated antigens are supposed to induce a T-cell-response, either in vitro or in vivo.
Thus, TAA represent a starting point for developing a tumor vaccine. Methods for identification and characterization of TAA are based on the utilization of patient-derived CTL or on the generation of differential transcription profiles between tumors and normal tissue.
Identification of genes which are overexpressed in tumorous tissues or which are selectively expressed in such tissues does not provide precise information about utilization of antigens transcribed from these genes for immune therapy. This is based on the fact that only several epitopes of these antigens are suitable for such an utilization since a T-cell response is induced—via MHC presentation—by epitopes of the antigens only and not by the antigen as a whole. Thus it is important to select those peptides of overexpressed or selectively expressed proteins, that are presented by MHC-molecules, thereby generating points of attack for specific tumor recognition by cytotoxic T-lymphocytes.